TNS-BUF Isolation Amplifier

The TAPR TNS-BUF is a very low noise,
high isolation, buffer amplifier for use in time and frequency measurement
applications. It's designed for use at 5 or 10 MHz, but is usable with
some loss of gain from 1 MHz to at least 50 MHz. The TNS-BUF circuit
was designed by Bruce Griffiths and John Miles provided valuable input
on both schematic and layout.

This page documents performance of the Rev. B prototype; production units
will be based on the Rev. C board, which is electrically identical.

The TNS-BUF is built on a 1.75 x 3.75 inch board using 0805 size surface
mount components. In the picture above, the SMA connectors are mounted
on the reverse side of the board.

Phase Noise

Here is a plot of the TNS-BUF phase noise using a 5 MHz ULN source.
The "bullet" trace shows the system noise floor with the TNS-BUF replaced
by an SMA bullet. (Click on the image for a full-sized version.)

In this plot, the TNS-BUF jumpers were set to 2-3, 2-3, 1-2 to yield
about 1 dB gain (see below for a complete table of gain versus jumper
settings). It's very important to use this rather than the
alternative 1-2, 2-3, 2-3 setting; that setting causes a phase noise
increase of more than 10dB.

Here is the Allan Deviation derived from the same data run:

And this shows the phase residuals:

Gain, Input Match, and Bandwidth

The TNS-BUF has three jumpers that set the gain of the unit from about
-10 dB to +7 dB. The gain settings have some impact on the return loss,
especially above 15 MHz. The following plots show the gain and return
loss with all jumpers set for maximum (yellow and cyan) and minimum
(green and red) gain.

This is the measured gain with various jumper settings:

GAIN_A

GAIN_B

GAIN_C

5 MHz

10 MHz

1-2

1-2

1-2

-11.0

-10.6

2-3

1-2

1-2

-5.0

-4.6

2-3

2-3

1-2

+1.0

+1.4

2-3

2-3

2-3

+7.0

+7.3

1-2

2-3

2-3

+1.0

+1.3

As noted above, the 1-2/2-3/2-3 setting has much worse phase noise,
because the gain is coming from the second and third, rather than the
first and second, stages. Putting the gain in the early stages maximizes
phase noise performance.

GAIN_A has the most impact on frequency response and return loss. From
the table above, it looks like gain settings 2-3/2-3/1-2 and 1-2/2-3/2-3
yield about the same gain at 5 and 10 MHz. But at 80 MHz the first setting
gives -1.2 dB gain and a return loss of 4.2dB, while the second -2.7 dB but
has a much better return loss of 13.9 dB. At 10 MHz, the return loss is
marginally better with GAIN_A set to the low position.
Relative gain and return loss at 1 MHz are not noticeably affected by the
GAIN_A setting; gain is down about 4 dB from that at 5 MHz and the return
loss is about 20 dB. The limiting factor at the low end of the range is
the performance of the four Minicircuits transformers, which are spec'd
for 1 MHz and above.

Reverse Isolation

A key requirement for many applications is high reverse isolation -- that
is, how much a signal going backwards through the buffer is attenuated.
High attenuation is required to avoid injection locking and other problems
in low noise measurements.

The theoretical reverse isolation of the TNS-BUF is well over 100dB, which
is a tough thing to measure. With all the stops pulled out, my old
8753C VNA can measure down to about -110dB. The TNS-BUF was at the
floor of that measurement. An alternative measurement using a signal
generator and spectrum analyzer shows an isolation at the minimum gain
setting of 111.8 dB, and at the maximum gain setting of
103.7 dB.

Given the high dynamic range required for this measurement, I don't want
to put a specific number out there until I can do further testing, but it's
safe to say that the TNS-BUF has at least 100-110 dB of isolation.